This is a guarenteed way to get a MASSIVE performance increase to your game, at the expense of taking up a little more hard drive space (2 gigs approx). I went from running low quality at 800x600 to running High Quality on 1024x768. This improves load times AND framerate/details.
There is a catch! By doing this, you temporarily can't connect to pure multiplayer servers. You have been warned (I'll tell you how to revert it at the end)
After installation, the Doom III Folder consists only of the doom3.exe and some .PK4 files. Those PK4 files includes the entire game (sounds, videos, textures etc.) compressed. They are normally uncompressed in mid game, which is very hard on your CPU. Lets fix this!
1. Make a copy of ALL the files in C:\Program Files\Doom 3\base. This is some big filage, but if you want to revert to MP without reinstalling you need to do it.
2. Unrar the following .pk4 files (just pretend they are .rar's and open them with winrar) into the C:\Program Files\Doom 3\base folder (each one should unrar a single folder such as textures or models):
pak004.pk4 (This one has many files, put them all in \Base and over right any that apply.
3. Now delete the files above (pak000-004) or move them to another folder for storage so you can revert to multiplayer.
When you want to revert to multiplayer, restore everything you had before and get rid of the new stuff. I'm not sure if the folder "maps" is the same or not, so make sure you back that up.
When in normal mode, you should have "maps" "savegames" "config.spec" "doomConfig.cfg" "doomkey" "game00.pk4" and "gamex86.dll", nothing else.
ALSO! If you open DoomConfig.cfg (C:\programfiles\doom3\base) in notepad and change
seta image_cacheMegs "32"
to a higher number, such as 96 or 128 you will see a big increase too. Go for it bad boys!
Try the new 4.9 beta drivers from ati 2